Currently, the propane-precooled mixed-refrigerant process developed by Air Products of the United States and the Tealarc process developed by Technip of France are widely used as the liquefaction processes for base load LNG plants. These two processes rely on the use of extremely large Hampson heat exchangers, but Hampson heat exchangers can be constructed only in plants equipped with special facilities, and are therefore expensive and require long periods of time to manufacture. Therefore, the need for such heat exchangers contributed to the increase in the costs for constructing LNG plants and the difficulty in enlarging existing LNG plants.
The applicants have therefore previously proposed a method for liquefying natural gas which minimizes the requirement of such expensive and special heat exchangers, and can be readily applied to LNG plants of a wide range of capacity in U.S. Pat. No. 5,363,655 issued Nov. 15, 1994. However, according to this method for liquefying natural gas, because the temperature range in the precooling unit is relatively wide, the refrigerant is required to have a large number of components, and the facility for producing the refrigerant tends to be costly. In particular, if the natural gas field produces little of a C5 fraction, the refrigerant cannot be produced within the LNG plant.